| Fusarium circinatum | |
|---|---|
| Pitch canker affecting western white pine | |
| Scientific classification | |
| Kingdom: | Fungi |
| Phylum: | Ascomycota |
| Class: | Sordariomycetes |
| Subclass: | Hypocreomycetidae |
| Order: | Hypocreales |
| Family: | Nectriaceae |
| Genus: | Fusarium |
| Species: | F. circinatum |
| Binomial name | |
| Fusarium circinatum Nirenberg & O'Donnell |
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| Synonyms | |
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Fusarium circinatum is a fungal plant pathogen that causes the serious disease pitch canker on pine trees. Like other Fusarium species in the phylum Ascomycota, it is the asexual reproductive state of the fungus and has a teleomorph, Gibberella circinata.
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This fungus is believed to have originated in Mexico. It spread to the eastern United States in 1946 and by 1986 had reached the western United States. It was first recorded in Japan in the 1980s, in South Africa in 1990, in Chile [1] and Spain in the mid 1990s and in Italy in 2007.[2]
In California this canker has been recorded on nine different species of pine (Pinus) and on Douglas fir (Pseudotsuga menziesii).[2] In Europe and Asia it has been recorded on over 30 other Pinus species.[3] Monterrey pine (Pinus radiata) seems to be the most susceptible species and in California 85% of the native Monterey pine forests may be killed within a decade.[4]
F. circinatum infects the twigs and branches of pine trees, causing a bark canker. Most infection is by macroconidia or microconidia. The macroconidia are 3-septate, with slightly curved walls and the microconidia are single-celled, ovoid, and borne in false heads on aerial polyphialides. The aerial mycelium is white or pale violet colour and slightly twisted below the proliferation of microconidiophores. In culture, perithecia are readily produced. They are dark purple or black and ovoid.[5] Cylindrical asci are released by oozing. There are eight ascospores which are 1-septate and ellipsoidal to fusiform. Because peritheca have not been observed in the field, it is not thought that ascospores are an important route for infection.[3]
The infection is usually carried from tree to tree by the rain, the wind or by bark-feeding insects. These including weevils in the genus Pityophthorus and bark beetles in the genera Ips and Conophthorus. These insects commonly infeat pine trees and the adults may disperse the pathogen. Additionally, these insects cause a wound when feeding and this may facilitate entry of the infection.[6] Warmth and moisture encourage the development of the disease whereas cooler drier conditions restrict it. In California it is more severe in coastal areas.[7]
Research was undertaken to see whether spores from the telemorph, Giberella circinata, might be responsible for spread of the fungus. It was found that very few vegetative compatibility groups existed among the California strains of the pathogen. This implied that asexual reproduction predominated and laboratory tests confirmed this.
In Chile, the infection was first reported on Pinus radiata in nurseries and was thought to be due to the import of contaminated seed. The symptoms were similar to other damping off diseases with seedlings wilting and dying and exuding resin from the root collar areas. Seedlings could also be infected by soil-borne contamination. A few years later, the disease had not spread to mature stands of trees.[1] The same is true in South Africa, where it was reported to infect nursery stock but not forest trees.[8]
F. circinatum is spread locally by wind and insects, but it is slow to spread into new areas. Over large distances it can be transported in contaminated pine seeds or by young plants. Although it could be carried as infected timber, this is considered unlikely especially if the bark has been removed. If timber had been a significant means of infection, the fungus would have spread more rapidly to other parts of the world as there is a considerable trade in pine.[3]
A test has been developed for detecting contamination of seeds of Pinus species by F. circinatum. It is based on the biological enrichment of a sample followed by a real-time polymerase chain reaction assay. Many countries are imposing quarantine controls on movement of propagating materials and this test can help prevent the introduction of the pathogen through contaminated seed.[9]